Question

what is the speed of beta particle

Answer 1

it depends on how much kinetic energy has it.

Answer 2

That having been said.

\[E = m _{0}c ^{2}/ \sqrt{1-\left( v/c \right)2}=m _{0}c ^{2}+KE\]

if the KE is equal to the rest mass of the beta particle for example ie KE =0.511 MEV, doing the math one finds that the velocity is 0.866c where c is the velocity of light (300,000,000 m/sec).

Answer 3

Less than the speed of light, but highly variable depending on the energy. A hot beta can be going at 90% of the speed of light.